A wave of assassinations in Benghazi has led to the day on which they happened being dubbed 'Black Friday'. Photograph: Abdullah Doma/AFP/Getty Images

Two popular young peace activists were among ten prominent Libyan civil rights workers and senior army officers assassinated in the eastern city of Benghazi on Friday, a day that has swiftly become known as Black Friday.

The coordinated killings, a vivid demonstration of the power of militants, took place at a time when the country is in chaos: an Islamist-led coalition is holding Tripoli and the UN has reported that 250,000 have fled the fighting.

Benghazi, Libya's eastern capital, is used to violence after a four-month campaign by a former general, Khalifa Hiftar, against an Islamist militia, Ansar al-Sharia, which is blamed by Washington for the killing two years ago of their ambassador, Chris Stevens. But Friday's mass killing is unprecedented.

The teenage campaigners, Tawfik Ben Saud and Sami Elkawafi, were known for their cheerful efforts to promote peace and democracy and were being mourned by friends yesterday. "I knew Elkawafi, he was a dedicated campaigner. This obviously has the fingerprints of Ansar al-Sharia," said Hasan al-Emin, a Libyan politician who was himself forced to flee to Britain after death threats from a militia. "To kill so many like this, it is really a disaster."

Other victims included five senior army officers; three more officers and a popular moderate imam survived assassination attempts. Some victims were shot from cars, others murdered at fake checkpoints. Ansar al-Sharia, which rejects democracy and proclaimed Benghazi an Islamic emirate in June, has yet to comment on the killings, which have caused sorrow on social media.

"Fighting the urge to just break down and sob. Benghazi drowning in blood and Tripoli drowning in chaos," tweeted Sarah Hussein, a student. An anonymous tweet read: "The reality is if you speak against Islamists in Libya you will get killed."

Libya's parliament – which was obliged to flee to the eastern town of Tobruk after the Islamist-led militia calling itself Libya Dawn captured the capital – called for international help at a conference of western and Arab leaders in Madrid last week. "We are in desperate need, we have a terrible humanitarian situation," said deputy speaker Mohammed Ali Shuhaib. "We want [outside powers] to understand the threat to their countries if Libya becomes a failed state. After the terrorists destroy us they will come for you."

Libya Dawn's leaders insist that they are not a terrorist group, saying they are true to the original aims of the 2011 revolution that overthrew Muammar Gaddafi, and that the newly elected parliament is loyal to Gaddafi.

Western powers, their hands full combating Islamic State (Isis) in Iraq and Syria, are reluctant to contemplate military intervention, but help for Libya's government is arriving. Last week Libya Dawn's positions near Tripoli were bombed by mystery jets: US sources told the New York Times that the jets are sophisticated bombers from the United Arab Emirates via bases in Egypt, something both states deny. But Cairo has warned that it will not tolerate an Islamist takeover of Libya.

Ansar al-Sharia dominates Benghazi but a ring of government forces around the city is tightening: the Islamists are running short of fighters and ammunition and being bombed by the Libyan air force. Some think militant weakness, rather than strength, is behind the mass assassinations, a warning to the government not to launch its threatened offensive to retake the city.

"What [Ansar al-Sharia] is trying to do is decrease pressure on them, especially from the air attacks," said al-Amin. "They are frightening the people who are inside the city."